Generate commands list as Markdown table

## Problem
Each `CliApplication` implements a list of commands.
This list can be printed in console output as part of usage help message.
However, it is usually required to have this list also in a documentation page.
The corresponding section of the doc page can thus be generated for any CliApplication instance.

## Solution
Implement the `genMarkdown` method which uses `commands` method for a list of supported commands.

[Example](https://github.com/ergoplatform/ergo-tool#supported-commands)